The present invention relates to a digital signal reproducing apparatus suitable for reproducing digital PCM audio signals that have been recorded in the form of single helical tracks on a recording medium, one track being formed per unit time, with a rotary head.
A technique is known in which audio signals are recorded on magnetic tape with a helical scanning a rotary head in the form of helical tracks, one track being formed per unit time, and reproduced thereafter. A digital signal record/reproduce apparatus known as R-DAT (rotary head type digital audio tape recorder) has been designated for recording audio signals as PCM signals and thereafter reproducing the same.
A format of tracks to be recorded in the actual system of R-DAT has a pattern as shown in FIG. 13(a), in which each of MARGIN, PLL and POSTAMBLE has a frequency or 1/2 f.sub.M (f.sub.M =9.4 MHz) and IBG a frequency of 1/6 f.sub.M. Each of SUB-1, SUB-2 and PCM is composed of a plurality of blocks as shown in FIGS. 13(a) and 13(b). SYNC is composed of 10 bits, 9 of which are fixed with the remainder assuming various patterns depending upon the place and audio signals. SUB consists of a cyclic pattern of 8 such blocks, and PCM 128 blocks. The numerals given in FIG. 13(a) represent the numbers of blocks occupied by the respective regions. ATF-1 between SUB-1 and PCM and ATF-2 between PCM and SUB-2 are each a region (ATF automatic track finding) provided for ensuring that tracking control i.e., control for allowing a rotary head to correctly scan the recorded tracks during reproduction, can be accomplished by means of the output of the head without employing any special head.
In R-DAT, PCM signals compressed on a time base are recorded in the form of helical tracks on magnetic tape by means of two rotary heads. Instead of providing a guard band between adjacent tracks, a tracking pilot signal is recorded both at the beginning and at the end of each track in a region independent of the area in which the PCM signals are recorded During reproduction, the recorded tracks area scanned with a rotary head having a scanning width larger than the width of each track, and the reproduction output of the pilot signals from the two tracks adjacent to the track being scanned is used to control the tracking of the rotary head.
The track pattern for ATF is specified as shown in FIG. 14 and is hereinafter described with reference to the case where a drum having a diameter of 30 mm is rotating at 2,000 rpm with the tape wound at an angle of 90.degree. to the drum.'
ATF-1 and ATF-2 located in the front and rear portions, respectively, of each track have a low-frequency (small azimuth-effect) signal f.sub.1 as a tracking pilot signal. This signal is used for the purpose of detecting the levels of crosstalk resulting from the two tracks adjacent to the track being reproduced, so as to obtain the difference between the levels of such crosstalk as a tracking error signal. A low-frequency signal of f.sub.M /72 (130 kHz) is used as the pilot signal f.sub.1.
In each of ATF-1 and ATF-2 is recorded a sync signal for identifying the location at which the pilot signal f.sub.1 is recorded. In the presence of crosstalk, the sync signal is unable to distinguish the on-track from adjacent tracks, so it is selected in such a way that it not only has a frequency capable of producing an azimuth-effect but also affords a pattern that is not possessed by the PCM signal. If the head having a +(plus) azimuth is designated A and the head having a-(minus) azimuth as B, two different sync signal are provided for the purpose of distinguishing head A from head B. Stated more specifically, a sync 1 signal f.sub.2 having a frequency of f.sub.M /18(=522 kHz) and a sync 2 signal f.sub.3 having a frequency of f.sub.M /12(=784 kHz), as associated for heads A and B, respectively, are recorded in predetermined positions.
In R-DAT which does not employ an erase head, a new signal is written over the previously recorded signal. In order to enable this "overwrite" mode, an erase signal f.sub.4 having a frequency of f.sub.M /6(1.56 MHz) is recorded at a predetermined position for erasing the previously recorded pilot signal f.sub.1, sync 1 signal f.sub.2, and sync 2 signal f.sub.3.
The pilot signals for ATF are located at different positions on the on-track and the two adjacent tracks and the level of the pilot signal on the on-track (i.e., the track being scanned) differs on a time basis from the level of each of the pilot signals on the adjacent tracks, so that the three different levels can be sampled independently of each other.
Five blocks are assigned to each of the ATF regions, ATF-1 and ATF-2, and the pilot signal f.sub.1 is recorded in two of the five blocks of each of the ATF regions. The sync signal f.sub.2 is recorded in an area covering 1 or 0.5 blocks beginning at the center of the position in which the pilot signal f.sub.1 of one of the two adjacent tracks is recorded. The pilot signal f.sub.1 on the other adjacent track is recorded in such a way that its center is positioned two blocks after the beginning of the sync signal recorded on the on-track. A sync signal composed of one block is assigned to an odd-number frame, and a sync signal composed of 0.5 blocks is assigned to an even-number frame.
As described above, the sync signals to be recorded in the ATF region have different frequencies depending upon which head is used in scanning, and these sync signals also have different recording lengths in odd-number frames and even-number frames. This design enables four consecutive tracks to be distinguished from one another since they are provided with different ATF regions. The pattern of ATF regions is of the a 4-track completed type in which it is cyclically repeated for every 4 tracks.
When magnetic tape in which signals have been recorded in the format shown in FIG. 13(a) is played back with a rotary head, an RF signal of the type shown in FIG. 15(a) is reproduced from the head. If this RF signal is obtained by playback of a track with the odd-number frame (A) shown in FIG. 14, it may be passed through a bandpass filter (BPF) of 130 kHz so as to obtain a pilot signal f.sub.1 as shown in FIG. 15(b).
The signal in zone I is due to the pilot signal on the on-track, and those in zones II and III result from the a crosstalk of the pilot signal on a track with the odd-number frame (B) and a track with the even-number frame (B), respectively. If the rotary head were scanning the on-track correctly, the envelope levels of zones II and III, or the values of V.sub.II and V.sub.III indicated in FIG. 15(c) should be equal to each other. However, if a tracking deviation occurs, V.sub.II is not equal to V.sub.III (V.sub.II .noteq.V.sub.III) and the amount and direction of the deviation of the rotary head with respect to the on-track can be determined by the magnitude and polarity of the difference between V.sub.II and V.sub.III. Therefore, by actuating a capstan servo according to the difference between V.sub.II and V.sub.III so as effect fine adjustment of the tape speed, the rotary head can be controlled to travel correctly on the on-track.
In practice, however, the performance of head A differs from head B and the condition under which one head is mounted on the drum also differs from the condition of mounting the other head. If such variations exist, the level of the reproduction with head A differs from the level for head B and as a result, the level of crosstalk of pilot signals comes to vary from one head to the other. This difference in the output level of crosstalk from the rotary heads causes the error signals for ATF tracking to vary from one reproduction head to the other, resulting in an exaggerated variation in the tracking error signal. This can be explained more specifically as follows: even if the two heads A and B have the same amount of mechanical deviation with respect to the tracks with which they come in contact for reproduction, the difference between the levels of crosstalk of pilot signals from the two tracks adjacent to the track being scanned fails to assume the same value for each head (this difference should inherently assume the same level for each head) if the two heads produce different output levels, and the capstan servo will act in different ways for each change of heads. As a consequence, control performed on one head in a certain direction may sometimes cause a greater amount of tracking deviation for the other head.
This problem could be solved by compensating for the tracking deviation with an offset being introduced in the tracking error signal for each head but then, this presents a great bottleneck in commercial production of DAT systems because an extra job of adjustment is necessary for setting a proper amount of offset for individual systems.